1. Field of the Invention
This invention relates to a process and apparatus for fluid catalytic cracking (FCC) of a hydrocarbon feed. More particularly, it relates to a process and apparatus for axially injecting lift gas through a lift pot having conduits therein to achieve a substantially uniform ratio of catalyst to oil across a horizontal cross-section of an FCC riser.
2. Discussion of the Prior Art
The field of catalytic cracking, particularly fluid catalytic cracking, has undergone significant developments due primarily to advances in catalyst technology and product distribution obtained therefrom. With the advent of high activity catalysts and particularly crystalline zeolite cracking catalysts, new areas of operating technology have been encountered, requiring refinements in processing techniques to take advantage of the high catalyst activity, selectivity and operating sensitivity.
By way of background, the hydrocarbon conversion catalyst usually employed in an FCC installation is preferably a high activity crystalline zeolite catalyst of a fluidizable particle size. The catalyst is combined with hydrocarbon feedstock (oil) and steam and is transferred in suspended or dispersed phase condition generally upwardly through one or more conversion zones (FCC riser), providing a hydrocarbon residence time in each conversion zone in the range of 0.5 to about 10 seconds, and usually less than about 8 seconds. High temperature riser conversions, occurring at temperatures of at least 980.degree. F. and at 0.5 to 4 seconds hydrocarbon residence time in contact with the catalyst in the FCC riser, are desirable for some operations before initiating separation of vaporous hydrocarbon product materials from the catalyst. Carbonaceous deposits accumulate on the catalyst particles, during the hydrocarbon conversion step, and the particles entrain hydrocarbon vapors upon removal from the hydrocarbon conversion step. The entrained hydrocarbons are subjected to further contact with the catalyst until they are removed from the catalyst by mechanical means or stripping gas or both in a separate catalyst stripping zone. Hydrocarbon conversion products, separated from the catalyst, and stripped materials are combined and typically pass to a product fractionation step. Stripped catalyst containing deactivating amounts of carbonaceous material, hereinafter referred to as coke, then passes to a regeneration operation.
Of particular interest has been the development of processes and systems for feeding oil, steam and catalyst into a FCC riser and improving the contact between the oil and catalyst to obtain uniform mixing as they travel up the riser.
U.S. Pat. No. 4,435,279 to Busch et al discloses charging low quality naphtha from thermal cracking either separately or in admixture with C.sub.5 and lower boiling wet gas product of hydrocarbon conversion to the bottom portion of a riser conversion zone for contact with freshly regenerated zeolite cracking catalysts. The vaporous material thus charged conveys the regenerated catalyst at an acceptable velocity to an expanded section of the riser wherein residual oil is charged by a plurality of nozzles penetrating the wall of the riser in the transition section to the expanded section.
U.S. Pat. No. 3,821,103 to Owen et al discloses a fluid catalytic cracking regeneration operation wherein hot catalyst, from a fluid catalytic cracking stripping vessel, passes to a vessel housing a bed of catalyst about the inlet of a riser regenerator. Regeneration gas, such as air or air supplemented with oxygen, is introduced, by a hollow stem-plugged valve aligned with the bottom open inlet of a regenerator riser.
While the systems described above mix a catalyst and a lift gas, there still remains a need for a system which obtains as nearly as possible a uniform ratio of catalyst to oil feed across the horizontal cross-section of a riser. The present invention is directed to filling this need.